Polarity

“A state or a condition of a molecule having positive and also negative charges, particularly in case of magnetic or electrical poles.”

All molecules have a permanent number of electrons which are arranged at certain energy levels called a shell. The electrons present in the valence shell are involved in chemical bonding with other atoms. Atoms tend to get the noble gas configuration to attain stability.  So, we can conclude that chemical bonding is responsible for the stability of atoms and form molecules. On the basis of participation of atoms and shifting of electrons, chemical bonds can be of different kinds like a metallic bond, covalent bond, ionic bonds etc. Ionic bonds have the electrostatic force of attraction between two oppositely charged ions. These ions are formed after shifting of electrons. When an atom receives an electron, it gets a negative charge and becomes an anion. In the same way, when an atom gives away an electron, it gets a positive charge and becomes a cation.  A cation and anion attract each other due to opposite charges and this electrostatic attraction force is known as an ionic bond. In the course of the formation of the ionic bond, electrons completely transfer to atoms, therefore, there are negative and positive charges on ions. Such types of bonds are established between metals and non-metals. Metals tend to release electron and form cation. On the other hand, non-metals tend to receive electrons and form anion. Unlike ionic bonds, covalent bonds are made by equal sharing of valence electrons of bonding atoms. In such types of bonds, the bonding atoms share an equal number of valence electrons with each other. These shared electrons are placed exactly at the core of chemical bonds, therefore, there is no charge on any of the bonding molecule. 

Covalent bonds are typically found between two non- metals or elements with similar electronegativity. For instance, the chlorine molecule is formed by equal sharing of electrons (one electron from each chlorine atom) from bonding atoms. Every covalent bond is made by sharing two electrons. If an atom needs more than one electron to get Stable configuration, it can share the same number of electrons to form covalent bonds. 

It results in the development of multiple covalent bonds like two oxygen atoms are bonded with double covalent bonds by the distribution of two electrons from each oxygen atom. In the same way, two nitrogen atoms are bonded with a triple covalent bond to get a steady nitrogen molecule.

What is Polarity in Chemistry?

We know there are many physical properties of compounds like density, melting and boiling points, solubility, volume etc. One of the significant properties of molecules is polarity. The polarity of molecules disturbs other physical properties of the molecules. The polarity of a molecule depends on the type of chemical bonding in the molecule and also on the bonded atoms.

If there is a clear separation of charges, we assume that there is polarity in the molecule. Polarity can be with an ionic and covalent bond. Several of the molecules have polar chemical bonds but are still non-polar in nature due to the equal arrangement of the chemical bonds. Polarity, in common, refers to the physical properties of compounds such as boiling point, melting points, and their solubility. The polarity of bonds mainly arises from the space between molecules and atoms with difference in electronegativities. Moving on, usually, the term Polarity is used in areas like magnetism, electricity, and signaling of electronic devices. Consider an electromotive force (EMF) or an electric potential, acting between two poles.. The pole having more electrons has a negative polarity whereas the other end has a positive polarity.

Discussing polarity in Chemistry, well it is mostly the separation of an electric charge which leads a molecule to have a positive and negative end. Consider the below illustration-

In an H-F bond, the fluorine atom is more electronegative than that of the Hydrogen atom. The electrons spend more time at the Fluorine atom. Therefore, this F atom marginally becomes negative whereas the Hydrogen atom tends to become slightly positive.

Polarity of Molecules

The bond or the molecular polarities relies upon the electronegativity of the atoms or the molecules. A molecule is mostly said to be either a polar molecule, non- polar molecule or ionic molecule.

Polar Molecules: A polar molecule is typically formed when the one end of the molecule is said to have the high number of positive charges and whereas the opposite end of the molecule has negative charges, generating an electrical dipole. When a molecule or atom is said to have a polar bond, then the center of the negative charge will be on one side, whereas the center of positive charge will be on the other side. The complete molecule will be a polar molecule.

Non- Polar Molecules: A molecule or atom which does not have any charges present at the end due to the reason that electrons are equally distributed and those which symmetrically cancel out each other are the non- polar molecules. In a solution, a polar molecule cannot be mixed with the non-polar molecule. For example, take water and oil. In this, water is a polar molecule whereas oil acts as a non- polar molecule. These two molecules do not form a solution as they cannot be mixed up together.

Polar and Non- polar Molecules Examples

• A molecule or atom may be polar or non- polar. A non- polar molecule has a configuration of its atoms lined up in a way that the orbital electrons are in the outer region canceling out the electronegativity.

• In common, pyramid-shaped and V-shaped molecules are called polar. Whereas the Linear molecules are said to be non-polar.

• Water is said to be a polar molecule due to the dissimilarity in the electronegativity between the oxygen atom and the hydrogen. Oxygen is an extremely electronegative atom when compared to hydrogen.

• Fats, petrol, oil, gasoline are known to be non-polar molecules as they do not dissolve in water and nonpolar are insoluble in water.

• Glucose is one more example of a polar molecule based on the configuration of the oxygen and hydrogen atoms in it.

Bond Polarity Example

Bond polarity signifies a separation of charge in a molecule. It can be calculated by the dipole moment of the chemical bond. If a chemical bond is formed between atoms with different electronegativities, the bonding electrons somewhat get shifted towards a more electronegative element. It induces slightly negative and positive charges over atoms. The polarity in the bond gives polarity to the molecule. For example; hydrogen chloride is a polar molecule because there is only one chemical bond that is polar in nature due to slightly negative and positive charges on bonding atoms.

The polarity in the bond is characterized by an arrow pointing towards the more electronegative atom.  

The total of polarity of all chemical bonds in a molecule gives the polarity of that molecule.

Factors on which the Polarity of Bonds Depend

• Relative Electronegativity of Participating Atoms or Molecules- Since the bond polarity involves dragging of electrons towards itself, a more electronegative element will be able to attract the electrons more towards itself. As a result, the electrons will absolutely move towards the more electronegative element. The amount of their transfer will depend upon the relative electronegativity of the participating atoms.

• The Spatial Arrangement of Various Bonds in the Atom or Molecule- The shared pair of electrons also experience dragging force from the other bonded and non-bonded pair of electrons. This results in different bond polarity among the same participating atoms that are present in other molecules. For e.g. Bond Polarity of O-H bond in an H2O molecule and acetic acid molecule is much different. This is due to the different spatial arrangement of many bonds in the molecule.

What are the Factors that Determine Whether a Molecule Is Polar?

• If the molecule or atom is perfectly symmetric, the molecule will not be polar even if there are polar bonds present.

• Polar bonds are formed when one atom in the bond has a much tougher pull towards electrons than the other atom. The difference in strength can be expected by comparing electronegativity values. If one electronegativity value is greater, that atom will pull the electron closer and develop a partial negative charge, while the other atom develops a partial positive charge.

Solved Example

1. C, H, O, N and S have the electronegativity 2.5, 2.1, 3.5, 3.0 and 2.5 . Among the following bonds, which is the most polar?
A) O – H      B) S – H      C) N – H       D) C – H

The difference in the electronegativity between two or more atoms is more; the bond among them is more polar. For the given atoms, we can see that:

• O – H = 3.5 – 2.1 = 1.4

• S – H = 3.5 – 2.5 = 1

• N – H = 3.0 – 2.1 = 0.9

• C – H = 2.5 – 2.1 = 0.4.

Hence, the O-H bond is the most polar among the given bonds.

Difference Between Polar and Non- Polar in Chemistry

Bonds are the main keyword that could be found all over in chemistry. The main backbone of school chemistry is the covalent bonds (sharing of electrons between two atoms) which could also be said as the base of the chemical reactions that occur between elements and atoms. There are two types of covalent bonds in chemistry namely polar covalent bonds and nonpolar covalent bonds.

Now, what’s the difference between polar and non- polar m. The basic difference between polar and non- polar is that polar bonds form a dipole-dipole bond or hydrogen bond which is the strongest among all other forces whereas nonpolar forms van der Waals force between them which is amongst the weakest forces. The electronegativity difference in the polar bonds is more than 0.4 whereas in non-polar bonds the difference is less than 0.4, the melting point in non-polar bonds is low in comparison to that of polar bonds. Similarly, the boiling point and surface tension are high in polar and low in non-polar only the polar bonds has low vapor  pressure whereas non-polar has high vapor pressure. These are the basic differences that can be easily learned and memorized as almost all the properties of polar and non- polar are opposite to each other. Students can also memorize the property of either polar or non- polar and can easily mention what would be the opposite property for the other one.